The present invention relates generally to methods and systems for consistently controlling system dynamics, and more particularly to a method and system for consistently controlling an active member connected to a spacecraft by decoupling the active member from the spacecraft.
Conventional methods for modeling active subsystems for spacecraft, such as a reaction wheel subsystem, involve modeling the system as two masses in which a first mass is a moving mass, the second mass is assumed to be ground, and an active control system controls the moving mass. For example, the first mass could be a levitated rotor and the second mass could be the rotor housing stiffly connected to the spacecraft. However, a problem can occur in the actual hardware of the control system when the spacecraft is not infinitely stiff or an infinite mass. This can cause instability in the controls and is undesirable.
This problem often occurs when an actuator commands an electromagnet to push (or pull) between a suspended rotor and a stationary housing to effect levitation. Accurate knowledge of the dynamics and mass properties of the levitated rotor and stationary housing as well as the spacecraft to which the stationary housing is attached is necessary to ensure control stability. However, the dynamics of the spacecraft model rarely fully converge to the dynamics of an actual spacecraft. Even if the dynamics of the spacecraft model are within the tolerance range of the actual spacecraft, a control system designed to operate correctly when bolted to one spacecraft may not operate correctly when bolted to another. In addition, the rotor within the suspended housing can transfer disturbance forces to the spacecraft. Such disturbance forces can hinder the control stability of the rotor within the suspended housing as well as input undesirable vibrations to the spacecraft.
The above-discussed problems are not limited to electromagnets. The disturbance forces and limited control stability can occur in other active control systems where mounting structures beyond the actual stationary housing and rotor may cause stability problems. This can occur when a structural modes and/or spacecraft related disturbances fall within the bandwidth of the control system. As a result, the conventional methods for modeling space structural systems and determining an actuator control system must account for structural (model) characteristics of a variety of spacecraft in determining the force of an actuator and the control loop for the actuator must be designed to react to low frequency disturbances (from the rotor) while not reacting to the higher frequency disturbances (from the spacecraft).
In view of the above, the present invention provides a method for estimating the dynamics of an active subsystem and determining a control force for the active subsystem by decoupling the active subsystem from a mounting surface (surface). A structural system is modeled as an active subsystem mounted to an isolation subsystem that decouples the active subsystem from a surface. The isolation subsystem decouples the active subsystem by a plurality (preferably six) of soft highly-damped isolators that connect the active subsystem to the surface and that provide highly damped isolation in a plurality (preferably six) of degrees of freedom. A control loop (in, for example, a microcomputer) commands the actuator to apply a control force to an active member within the active subsystem in order to maintain it at a specific bearing gap. The control force is determined based upon transfer functions of the active and passive subsystem, without having to take into account the dynamics of the surface. An actual structural system can subsequently be designed based upon the control force.
In a second embodiment, the active subsystem is modeled as an array of interconnected stationary housings. The interconnecting of the stationary housings provides more mass for the actuators to push against for maintaining the active members at the bearing gap within the active subsystem.
The present invention consequently enables an appropriate control force to be determined for application to an active member within a stationary housing without having to take into account the dynamic characteristics of the mounting surface.